A lifting mechanism for a nursing bed is described in DE 198 54 136 A1. The disclosed lifting mechanism included a frame that forms the pedestal of the lifting mechanism, as well as an upper frame that is arranged approximately congruent with the frame and serves as the lifting head. A total of four levers are provided in the side of the lifting mechanism. Two of the levers are respectively connected to one another by a toggle link. At the toggle links, two toggle lever pairs are kinematically connected to one another by a horizontal coupling brace. Another coupling brace connects one of the upper levers to a lower lever on each side, for example, the upper lever at the foot end of the bed to the lower lever at the head end. One end of the driving motor for moving the lifting mechanism engages a cross brace of the pedestal and the other end engages a connecting brace that connects the two horizontally extending coupling braces to one another.
Viewed from the side, the two lower levers and the horizontal coupling braces form a rod parallelogram that is raised by the spindle motor. The two upper levers also form a parallelogram together with the upper frame or lifting head. This latter parallelogram is raised via the diagonally extending coupling brace.
Due to this kinematic arrangement, an extremely high pressure force acts in the diagonally extending coupling brace. This pressure force does not depend at all on the patient's center of gravity being situated in the vicinity of the levers at the foot end or the levers at the head end. Enormous pressure forces act in the lower lifting range. Since the lower parallelogram is raised by the spindle motor, the forces that need to be generated by the motor are comparatively high when the lifting mechanism is lowered and decrease significantly as the parallelogram is raised. The transmission ratio in the lower lifting range is approximately 2:1, i.e., the lifting mechanism is stretched by twice the amount by which the motor is displaced. In contrast, the lifting ratio is reversed in the upper range. As a result, the lifting mechanism can only be raised very slowly in the upper lifting range.